Dislocation-free growth of silicon semiconductor crystals with &lt;110&gt; orientation

ABSTRACT

The disclosure relates to the growth of semiconductor single crystals, i.e., either by the pedestal method or by the Czochralski method, free from dislocations, particularly in the difficult directions of growth, such as &lt;110&gt;. The method causes dislocations parallel to the growth axis to grow away from the axis under the influence of a changing crystal diameter. The thickness of the stem portion of the crystal is controlled so that it has first a long thin portion to get rid of dislocations on the {111} planes that are inclined to the &lt;110&gt; axis and then a portion in which the residual dislocations are permitted to terminate at the surface. The latter portion may be generated in several ways. One way is to follow the long narrow diameter portion with a section of substantially larger diameter followed by a further narrow diameter portion. This combination is repeated one or more times to insure freedom from dislocations in the resulting crystal. By having the alternate thin and thick regions, the dislocations are gradually moved away from the axis of the crystal and toward the periphery where they will have a greater probability of terminating on a surface, thereby removing the dislocations.

This is a continuation of application Ser. No. 396,654, filed Sept. 12,1973, now abandoned.

This invention relates to the growth of semiconductor single crystals,either by the pedestal method or by the Czochralski method, and moreparticularly to a method of increasing the yield and certainty ofgrowing a dislocation-free crystal with <110> or any otherdifficult-to-grow orientation. In the following discussion, siliconcrystals of the <110> orientation are used for illustration.

Silicon slices of {110} orientation have inherent attractive featuresfor semiconductor device design. For example, ion implantationtechniques for making semiconductor devices can utilize the fact thatthe ion penetration is more effective in the <110> direction in thediamond lattice. Also, the {110} orientation provides superior devicepacking densities as compared with the {100} and {111} orientations.Furthermore, the {110} orientation possesses boron diffusioncharacteristics that are the same as on the {111} orientation. Thoughthese advantages of <110> crystals for various purposes have been known,it is also necessary in many applications that the crystal be free fromdislocations. This has presented a problem in the prior art because the<110> crystals have been plagued by poor yields due to suchdislocations, there being a factor of about 25 to 1 in the yield betweenthe <111> or <100> orientations as compared with the <110> orientationcrystals. These poor yields of the <110> crystals are believed to becaused by the propagation of the dislocations parallel to the <110>growth axis from the seed through the stem and into the main body of thecrystal where the dislocations can multiply to unacceptable densities.Prior art methods of removal of dislocations from silicon crystals, suchas those described in U.S. Pat. Nos. 3,275,417 and 3,397,042, do notremove all of the dislocations. Thus, although dislocation-free growthof <110> oriented silicon crystals is taught and accomplished in theprior art, the yield is low and is not as predictable as in the case of<111> and <100> growth.

It is well-known in the prior art that the <111> and <100> crystalorientations, the dislocations are removed in the growth process becausethe <111> planes on which they lie terminate on the surface of the stemportion of the crystal before they can multiply and propagate to otherplanes. Thus, the <110> direction of growth has at least one additionaldifficulty beyond those in <111> and <100> growth, there existing planeson which dislocations can lie parallel to the growth direction. Also,there is a preference for the dislocations to lie in any of three <110>directions in each of the {111} planes parallel to the <110> growthaxis, so that there is an energy barrier to overcome to divert suchdislocations away from the <110> growth axis.

Briefly, in accordance with the present invention, there is provided amethod for providing silicon crystals with <110> orientation having highyield as compared with the yields of the prior art methods. Briefly, inaccordance with the method, dislocations parallel to the growth axis aremade to grow away from the axis under the influence of a changing thindiameter. Continual enlarging and diminishing of the stem diametercauses the dislocations to be moved away from the axis and ultimately tomove sufficiently far from the axis until they terminate upon an edge ofthe crystal and therefore disappear. Though the number of dislocationsincreases due to the diameter change, it has been found that the rate ofdiscontinuity increase is less than the rate of removal ofdiscontinuities by this method. It is therefore possible to providesilicon crystals of <110> orientation having relatively high yield andwhich are substantially dislocation-free.

It is therefore an object of this invention to provide a method forproviding dislocation-free growth of silicon crystals of <110>orientation with relatively high yield.

It is a further object of this invention to provide dislocation-freegrowth of silicon crystals with <110> orientation by providing continualand alternately enlarged and diminished diameter areas in the crystalstem during crystal growth.

It is a further object of this invention to provide a high yielddislocation-free growth of silicon crystals with <110> orientation bycausing the dislocations on planes parallel to the stem axis to becontinually diverted away from the axis until they finally terminate onan edge of the crystal.

The above objects and still further objects of the invention will becomeapparent to those skilled in the art after consideration of thefollowing preferred embodiment thereof, which is provided by way ofexample and not by way of limitation, wherein:

FIG. 1 shows a mechanism of removing residual dislocations in a <110>silicon growth by a single widening and thinning of the stem of thegrown crystal;

FIG. 2 shows a mechanism for removing residual dislocations in <110>silicon growth by utilizing a plurality of widening and narrowingoperations on the stem of the grown crystal for removing thedislocations;

FIG. 3 is a schematic diagram of a construction for allowing terminationof dislocations in the <110> silicon growth on the edge of the crystalutilizing an RF coil to distort the shape of the original melt crosssection.

In accordance with the present invention, the stem portion of thesilicon crystal during crystal growth is controlled so that it has firsta long thin portion to remove dislocations on {111} planes that areinclined to the <110> axis and a portion in which the residualdislocations are permitted to terminate at a surface. The latter portionmay be generated in several ways. One way is to follow the long narrowportion with a section of substantially larger diameter followed by anarrow portion. This combination may be repeated one or more times toinsure freedom from dislocations in the resulting crystal. According tothis method, the residual dislocations on {111} planes parallel to the<110> axis are attracted to or change direction toward the periphery ofthe crystal. Then, a reduction in diameter allows the dislocations on{111} planes that are parallel to the growth axis to terminate where the{111} planes of conformity terminate on a surface. Such an operation isshown in FIG. 1 wherein a residual dislocation on a plane parallel tothe <110> growth axis 2 is numbered 1. This dislocation 1 travelsparallel to the axis of the crystal in the narrow stem portion 3. As thecrystal diameter is enlarged in the region 5, the dislocation growth ormovement is away from the axis 2 of the crystal and toward the surfacethereof. When the diameter of the crystal ceases to be enlarged, such asat the midpoint of the enlarged diameter portion 5, the dislocationagain travels parallel to the axis of the crystal and terminates at theedge 7, this causing removal of dislocation farther along the crystal.

Referring now to FIG. 2, there is shown a method wherein severalvariations in diameter are required. Here, a dislocation 11 is travelingparallel to the <110> growth axis 12 in the narrow stem portion 13. Thediameter of the crystal is then enlarged in the region 15 and causes thediscontinuity 11 to alter its path away from the axis 12 of the crystal.This movement is shown at the region 17. When the diameter of thecrystal is again reduced at the region 19, the dislocation again travelsparallel to the axis of the crystal, but closer to the edge thereof. Itcan be seen that the movement of the dislocation toward the surface atthis point has not been sufficient to cause termination thereof on anedge. A further enlargement of the cross-section in the region 21 againcauses the path of the dislocation to travel closer to the edge of thecrystal along the path 23 and then travel parallel to the axis of thecrystal again as the crystal diameter is narrowed in the region 25. Thecrystal diameter is again enlarged at the region 27, causing the path ofthe dislocation in the region 29 to again be moved toward the edge ofthe crystal and finally terminate in the edge of the crystal at thepoint 31. It can be seen in accordance with the method depicted in FIG.2 that several steps of enlarging and narrowing of the crystalcross-sections are required to gradually move the path of propagation ofthe dislocation away from the axis of the crystal and toward the edge toprovide ultimate termination of the dislocation in the surface of thecrystal.

The method depicted in FIG. 2 can be accomplished in another manner,this being to shift the solid portion of the stem relative to the liquidportion one or more times during the stem growth procedure. Thisproduces jogs in the stem portion which will automatically provideplaces for termination of the dislocations.

Referring now to FIG. 3, there is shown another embodiment of theinvention, wherein an RF coil 41 surrounds the crystal 43 which is beingpulled from the melt. The crystal 43 shown in phantom is heated by theRF coil, whereby the cross-section thereof is constricted as shown bythe crystal 45. This constriction and subsequent thickening of thecrystal stem will cause dislocations to move away from the axis in thesame manner as shown in FIG. 1 or at several points as shown in FIG. 2to move the path of the dislocation away from the axis of the crystal sothat it finally terminates at an edge and is removed.

It can be seen that in accordance with the methods set forthhereinabove, there are provided dislocation-free silicon crystals with<110> orientation, which can be produced with relatively high yield ascompared to prior art methods.

In a specific example, a <110> silicon crystal was grown which wasdislocation-free in which the stem section had, first, a 28millimeter-long thin portion of about two and a half millimeter diameterand then a portion of maximum diameter of about 8 to 9 millimetersfollowed by a section which was narrowed to two and a half millimeters.The stem was divided into sections and these were mechanically lapped,chemically polished and etched for dislocations. It was found that afterthe first narrow section of the stem only two dislocations remained.These were located at 0.7 millimeters from the center and continued tothe top of the third section where the stem was starting to bulge andwere located 1.0 millimeter from the center. At the bottom of the thirdsection the dislocations had multiplied to five, but were 1.4millimeters from the center. At the top of the fourth narrowing downsection, these five dislocations continued and were one and a halfmillimeters from the center on the average. Only two remained at thebottom of this section and they were very close to the edge of thecrystal. They were 1.3 millimeters from the center. At the top of thefifth section, only one dislocation remained at 1.0 millimeter from thecenter, close to the edge. At the bottom of this section, nodislocations were present and all the remaining sections weredislocation-free.

From this example, it is clear that the method described above waseffective in allowing the residual dislocations to terminate even thoughthey had multiplied from 2 to 5 in going through the bulged area.

Though the invention has been described with respect to specificpreferred embodiments thereof, many variations and modifications thereofwill immediately become apparent to those skilled in the art. It istherefore the intention that the appended claims be interpreted asbroadly as possible in view of the prior art to include all suchvariations and modifications.

What is claimed is:
 1. In a method of forming silicon crystals with<110> crystallographic orientation, which comprises the steps of:pullinga <110> crystal silicon stem having a long thin portion from a siliconmelt, said crystal having a first diameter and dislocations propagatingin a direction substantially parallel to the stem axis, said portionbeing sufficiently long to remove dislocations on {111} planes that areinclined to the <110> axis. the improvement comprising (a) enlargingsaid crystal stem to a second diameter sufficiently great to divert thedislocation propagation path away from the stem axis, b. then reducingsaid crystal stem to a third diameter sufficiently small to cause saiddislocation to terminate at a crystal edge, and then c. growing theremainder of the crystal.
 2. A method as set forth in claim 1 whereinsteps (a) and (b) are repeated until substantially all dislocations areterminated at a crystal edge.